Does he have permission to participate in activities like this? I haven't waterskied in years, but if memory serves, it's not light exercise. The Afib itself makes me dizzy, plus 3 of my medications. I agree with bbxx, it would be a good thing to see his doctor and get a clear idea of what's ok and what's not in the way of exercise.
First of all, 70 years old just learning to water ski? I am impressed!
Since he has heart issues, he really should be seen.